Create a Plugin structure for migration source files

[rvilar]

At the moment, the module have support for CSV and Yaml source files. This support is explicity in the code of the module and the addition of an extra source file implies adding new code in the module.
We want to add a Plugin Manager to add source files as Plugins and give the ability to other modules to provide their own source file importer.

[rodrigoaguilera]

Namely JSON ;)